After a setback, federal prosecutors are fighting to salvage their aggressive economic espionage case against four Chinese corporations accused of paying close to $30 million for stolen DuPont trade secrets.

But U.S. District Judge Jeffrey White still seems skeptical the government has lawfully served the Chinese entities under the rules of federal criminal procedure. Prior to hearing Thursday, White issued a tentative ruling quashing service of indictment against two of the four companies. He did not rule from the bench or indicate his views as to the other two entities.

White quashed service of indictment against the corporate defendants last year, sending the Justice Department back to the drawing board. Prosecutors have since tried again to serve summonses on the four firms that make up China’s Pangang Group, mailing the notices to last-known U.S. addresses for the corporations and even sending FBI agents to a conference on plastics in Louisville to confront a Chinese executive thought to work for Pangang.

However, defense attorneys at Quinn Emanuel Urquhart & Sullivan dispute that man has ties to the Pangang Group and insist the government’s second attempt at service is no more valid than its first.

"After these renewed efforts at service, the government remains in the same place that it started: none of the Pangang Group defendants have been served," Quinn partner Robert Feldman wrote in a brief.

The conundrum comes in a case where U.S. prosecutors took a hard line against alleged industrial spying by China and could signal an obstacle to similar prosecutions even as the U.S. government is raising the alarm that such conduct is widespread.

Prosecutors say Oakland, Calif., engineer Walter Liew, represented by Keker & Van Nest, sold the Pangang Group DuPont’s proprietary design for the production of titanium oxide, an industrial pigment used to give paper, paint and other commercial products a bright white color.

Under the Federal Rules of Criminal Procedure, the government must serve an organizational defendant by delivering a summons to a representative of the entity and also mail the summons to the defendant’s last-known domestic address.

Assistant U.S. Attorney John Hemann said it would be absurd that a foreign corporation could evade criminal prosecution by simply not maintaining a U.S. mailing address.

"We shouldn’t go about immunizing foreign corporations," said Hemann, who is prosecuting the case with Assistant U.S. Attorney Peter Axelrod. The Chinese government refused to serve the summonses on the Pangang defendants, Hemann said.

According to Quinn, the state-owned Chinese enterprise does not maintain any office in the United States where the summons could properly be mailed, and the attempted service in Kentucky was a case of mistaken identity. The Chinese chemical executive attending the conference was affiliated with an "imposter" company also named Pangang that is based in Hong Kong, according to Quinn.

The prosecution of Liew; his wife, Christina; and two former DuPont employees in connection with trade secret theft is not expected to be disrupted by the legal technicality over service.

On Wednesday prosecutors scored a win when White ordered Liew to remain in custody pending trial. The 55-year-old San Francisco Bay Area resident, a U.S. citizen, has been detained since his arrest in July 2011. A trial date has not been set.